Remove libtool to fix alpine 3.7 docker bug

Alpine 3.7 libtool pkg depends on bash pkg, this caused the compile hangs at "checking that generated files are newer than configure...", after check [this thread](https://forums.docker.com/t/problem-of-hanging-autoreconf/41015/3), I remove libtool and it works.

docker/alpine/Dockerfile

@@ -23,7 +23,6 @@ RUN set -ex && \
                                 build-base \
                                 curl \
                                 libev-dev \
-                                libtool \
                                 linux-headers \
                                 libsodium-dev \
                                 mbedtls-dev \